Relaxins are heterodimeric peptide hormones composed, in their mature form, of an A chain and a B chain linked via disulfide bridges. Human relaxins in their mature form are typically stabilised by three disulfide bonds, two inter-chain disulfide bonds between the A chain and B chain and one intra-chain disulfide bond between cysteine residues in the A chain.
Relaxins have been conserved through vertebrate evolution and have been characterised in a large and diverse range of vertebrate species. In particular the cysteine residues in the B and A chains responsible for the intra- and inter-chain disulfide bonds are highly conserved. Whilst in most species only two forms of relaxin have been identified (relaxin and relaxin-3), in humans three distinct forms of relaxin have been described and the genes and polypeptides characterised. These have been designated H1, H2 and H3, Homologues of H1 and H2 relaxin have been identified in other higher primates including chimpanzees, gorillas and orangutans. Differing expression patterns for H1, H2 and H3 relaxin suggest some differences in biological roles, however all three forms display similar biological activities, as determined for example by their ability to modulate (stimulate or inhibit) cAMP activity in cells expressing relaxin family receptors, and accordingly share some biological functions in common.
The biological actions of relaxins are mediated through G protein coupled receptors. To date, H1, H2 and H3 relaxins have been shown to primarily recognise and bind four receptors, RXFP1 (LGR7), RXFP2 (LGR8), RXFP3 (GPCR135) and RXFP4 (GPCR142). Receptors RXFP1 and RXFP2 are structurally distinct from receptors RXFP3 and RXFP4, yet despite the differences there is significant cross-reactivity between different native relaxin molecules and different receptors.
Initially thought to be predominantly a reproductive hormone, it has become increasingly clear that human relaxin-2 has pleiotropic actions. Relaxin-2 has been shown to have potent cardioprotective (including vasodilatory and angiogenic) effects and antifibrotic effects (see, for example, Du et al., 2010, Nat. Rev. Cardiol, 7, 48-58 and Samuel, 2005, Clin. Med. Res. 3, 241-249). Relaxin-2 is currently undergoing clinical trial evaluation for the treatment of acute heart failure.
With the increasing therapeutic promise shown by relaxin-2 and the continued development of potential clinical applications there is also an interest in developing relaxin peptides that are simpler in structure than native relaxin molecules and yet which retain the ability to bind to relaxin receptors and/or retain relaxin-associated biological activity. Simplifying the structure of therapeutic peptides and minimising the amino acid sequence required to impart biological activity on therapeutic peptides can serve to reduce the cost of polypeptide synthesis, reduce the complexity and difficulty of synthesis, and/or improve the efficiency of synthesis. Moreover, simplified, smaller molecules may exhibit improved in vivo activities and/or cellular uptake of such molecules may be improved when compared to native counterparts. In addition, improvements to pharmacokinetic properties (such as half-life, bioavailability etc) and/or therapeutic efficacy may be more readily made to simplified, smaller peptides.